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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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470                                ALASKAN GLACIER STUDIES
between Latouche Island on the west and Orca Inlet on the east. The slope of the former ice surface between the northern and southern points of determination is (1) about 25 feet to the mile from Harriman Fiord to Latouche Island, the grade flattening in the southern portion and being only about 17 feet to the mile between Knight and Latouche Islands; (2) about 60 feet to the mile between Columbia Glacier and Hinchinbrook Entrance; and (8) about 50 feet to the mile between Orca Inlet and Hinchinbrook Entrance. These grades are not quite as steep as those of the present Malaspina Glacier and of the ancient ice surface of Yakutat Bay, which average 70 to 75 feet to the mile. It will be noted that the surface of the piedmont glacier not only sloped southward from the northern snowfields toward the Pacific Ocean but also inward from, the eastern and western snow-fields to a low axis in the middle of the sound. This low axis was occupied by an ice stream which probably moved with some little rapidity, because it led to an outlet at Hinchinbrook Entrance.
Basins and Submarine Channels. The bottom of Prince William Sound, 200 to 400 fathoms deep, has been sculptured into basins and submarine channels by the movement of ice currents of the former piedmont glacier.1 These channels show a direct relationship to the tributary fiords. Our knowledge of the submarine topography is based upon soundings by the U. S. Coast and Geodetic Survey, supplemented by our own, the data being complete except for the northwestern corner of the sound between Knight Island and the mouth of Passage Canal—Port "Wells.
The largest channel (Fig. 72) leads from Columbia Bay and Valdez Ann to Hinchinbrook Entrance. It is about forty miles long and 7 to 10. miles wide, with an average depth of 1200 to 1500 feet. As the adjacent portion of the sound has a depth of only 400 to 700 feet it is evident that this channel is about 800 feet deeper than the portion of the sound through which it passes. At its head the expanded ice streams of Valdez Arm and Columbia Bay formerly came in from the northeast, joining a western ice stream from between Glacier Island and the Naked Island group, where the Wells Bay and Unakwik Inlet Glaciers evidently supplied part of their ice.
Ice streams from Port Fidalgo and from Port Gravina—Orca Bay came into the channel from the east and it is these that furnish proof as to the origin of this submarine channel. Port Fidalgo has a hanging valley relationship to this channel as does the united Port Gravina—Orca Bay. The different amounts of discordance, Port Fidalgo between 600 and 700 feet, Orca Bay 900 to 1000 feet, shows that these submerged hanging valleys are due to glacial erosion and that the great submarine channel itself is the result of ice sculpture. It also has the ice-eroded basin character, being 400 feet shallower near Hinchinbrook Island than it is a few miles farther north.
West of this channel and of northern Montague Island is a submarine ridge about ten miles wide and with water averaging less than 400 feet in depth. Above this ridge rise Naked and adjacent islands, Green Island, and a few small reefs.
A second submarine channel leads southward from between Naked and Knight Islands to Montague Strait, following close to the eastern shore of Knight. Island. It is about thirty-five miles long and three miles wide and is 400 to 800 feet deep. Its relative narrowness and shallowness is due to the small amount of ice which moved down it, most of which probably came from Unakwik Inlet and Eaglek Bay and from local glaciers on Knight Island. Its glacial origin is shown by the fact that its northern end hangs
1 Bean, E. F., Fiords of Prince William Sound, Alaska, Unpublished thesis, University of Wisconsin, 1011.